Sex allocation in a monogamous bird: advantaged matrilines and sons with silver spoons

The ability to secure food for offspring and withstand the cost of reproduction favors high-quality mothers that overproduce the larger sex, typically sons, only if they will receive adequate food as this should enhance these sons’ fitness returns. However, high-quality daughters ensure grandoffspring receive quality parental care and may possess greater reproductive value than their brothers, favoring daughters also from high-quality mothers. Using a mixed cross-fostering approach, we investigated effects of early rearing conditions, covariance between breeders and their genetic parents in parental quality, and primary offspring sex ratios in Carolina wrens. In this socially and genetically monogamous bird, sons grew larger than daughters, paternal food-provisioning impacted the condition and recruitment of sons, but not daughters, and females overproduced sons when paired with males that provisioned at a high rate, reflecting females’ anticipation of the quality of the rearing environment. Components of reproductive potential, including total fecundity, were inherited matrilineally and, all else being equal, females in better condition produced larger-than-average clutches biased toward daughters, who eventually produced larger-than-average clutches themselves. Sex ratios therefore varied with components of parental investment, but in opposing directions for matrilineally inherited and environmental effects, suggesting that multiple, countervailing selective forces shape sex-ratio variation.

Methods
All authors assisted with data collection. EKB wrote statistical code. These data were collected from 2017-2021.

具备为后代获取食物并承受繁殖代价能力的优质雌亲，会受到自然选择的青睐；这类雌亲会过量生产体型更大的后代性别——通常为雄性——但前提是这些雄性子代能够获得充足食物，因为充足食物可提升雄性的适合度收益。然而，优质雌性后代可确保孙代获得高质量的亲代抚育，且其繁殖价值可能高于雄性同胞，因此优质雌亲同样会倾向于过量生产雌性后代。本研究采用混合交叉抚育（cross-fostering）实验方法，以卡罗苇鹪鹩（Carolina wrens）为研究对象，探究了早期抚育环境的影响、繁殖亲鸟与其遗传亲本在亲代质量上的协方差，以及后代的初始性比。在这种社会与遗传双单配制的鸟类中，雄性体型大于雌性；雄性亲代的食物投喂行为会影响雄性后代的体况与种群招募成功率，却对雌性后代无显著作用；当雌性与投喂频率较高的雄性配对时，会过量生产雄性后代，这反映出雌性亲代会预判抚育环境的质量。包括总繁殖力在内的繁殖潜力组分呈母系遗传特征；其他条件均保持一致时，体况更佳的雌性会产下窝卵数高于种群平均水平且性比偏向雌性的窝卵，而这些雌性后代自身最终也会产下高于平均水平的窝卵数。因此，性比会随亲代投资组分发生变化，但母系遗传效应与环境效应的作用方向相反，这表明多种相互制衡的选择压力共同塑造了性比变异。

方法
所有作者均参与了数据采集工作。EKB 编写了统计代码。本研究的数据采集时段为2017年至2021年。